1. Field of the Invention
The present invention relates to glass sheet tempering apparatus and particularly relates to the tempering of large glass sheets, especially those that are shaped prior to being tempered. When glass sheets are tempered, each glass sheet in turn is heated above its annealing range and then rapidly cooled to set the surfaces of the glass sheet while the center is still hot. This action results in the sheet having its surfaces stressed in compression while the intermediate portion is stressed in tension after its temperature equalizes throughout its thickness.
The stress pattern imparted to tempered glass results in a much stronger sheet than untempered glass, because the glass surfaces, by virtue of being stressed in compression, are much more able to withstand external forces than untempered glass sheets which are not provided with such large compression stresses in the surface area. Moreover, when the outer surface of the glass is penetrated, tempered glass breaks up into small, relatively harmless, smoothly surfaced particles. In contrast, annealed glass fractures more readily, and when fractured, breaks into relatively dangerous, large, jagged fragments.
The uniformity of the size of the shattered particles indicates the uniformity of temper of the glass. The smaller, smoother particles of shattered tempered glass are much safer than the jagged fragments of untempered glass.
More specifically, in a typical tempering operation, a glass sheet is heated to nearly its softening point and then quickly chilled by uniformly exposing the opposite surfaces of the heated glass sheet to cold streams of a tempering fluid, such as air, arranged to cool both surfaces uniformly and simultaneously. The fluid is disposed through two opposed, spaced plenum chambers via nozzle boxes, each provided with an apertured wall through which extend a set of nozzles. The sets of nozzles face opposite major surfaces of the glass sheet.
The prior art considered it a prerequisite to uniform tempering to have an even distribution of the cooling air over the glass surfaces. This is usually accomplished by blasting air through a plurality of identical, uniformly spaced, elongated nozzles extending through apertures in apertured walls of the plenum chambers or nozzle boxes. The nozzles and/or the glass sheets are either moved in closed orbits or reciprocated through an amplitude sufficient to insure that each increment of the glass sheet area is swept by at least one of the nozzles. This distance between the nozzle orifices and the adjacent sheet surfaces have been kept as uniform as possible in order to strive for the goal of uniform tempering of the glass sheet.
It is necessary to impart relative movement between the nozzles moving in unison relative to the glass sheet to avoid nonuniform cooling of the glass. When the nozzles are not moved relative to the major glass surfaces or vice versa, the tempering medium blasts are directed against fixed locations on the glass. Fixed air blasts cool the fixed locations opposite the blasts rapidly while other locations adjacent to the fixed locations are not cooled as rapidly. Without such relative movement, patterns of iridescence form on the surface of the tempered glass. These patterns of iridescence are very annoying when viewed in reflection or in polarized light.
By providing relative movement of the nozzles relative to the major surfaces of the glass sheet, and by applying the streams of air or other tempering medium through the nozzles by pressure from a common source, prior art tempering apparatus provided substantially uniform tempering for flat glass and gently curved glass of relatively small and intermediate sizes. However, as the size and/or shape of automobile backlights and sidelights became larger and more complicated, it has become more and more difficult to temper glass sheets adequately. It has become necessary to supply air or other tempering medium at a greater rate of flow per unit area for larger sizes than for smaller sizes in order to assure that the glass is adequately tempered.
The glass sheet tempering art has developed many techniques for imparting relative motion between the nozzles that face the opposite surfaces of the glass sheet and the major surfaces of said sheet. Some of these involve linear reciprocation of the nozzles in unison. Others involve linear reciprocating or translational movement of glass sheets past a pair of arrays of fixed opposing nozzles. Others involving applying elliptical or circular orbital movement of nozzles relative to a glass sheet supported at a fixed position.
The prior art recognized that one of the problems of inadequate tempering of large glass sheets and/or those having complicated curvatures resulted from the inability of the air blasted against the central portion of the glass sheet to escape from between the central portion of the glass sheet and the apertured walls of the plenum chambers or nozzle boxes so as to enable fresh cool tempering medium to replace the spent tempering medium that impinged on the glass. The prior art recognized the correlation of the long escape path from the center to the edge of the glass sheet with inadequate center portion temper. According to one proposal to solve this problem, the wall of each plenum chamber or nozzle box facing the central portion of a glass sheet undergoing cooling has a greater proportion per unit area apertured than the remainder of the wall facing the portion of the glass sheet surrounding the central portion. Such a construction causes a slight pressure gradient in the tempering medium along the major surfaces of the glass sheets undergoing cooling from the central region to the outermost regions of the glass sheet surfaces facing the opposing sets of moving nozzles through which tempering medium is applied for tempering. This slight pressure gradient results in a continuous outward flow from the central portion of the glass to its entire peripheral margin and helps remove air from the vicinity of the glass sheet surface after the relatively cool air supplied through the apertured wall of the plenum chamber has engaged the heated glass surface to chill the latter and has in turn been heated by said engagement.
In the past, relative movement between the glass sheets and the tempering nozzles involved either limited relative movement of the tempering nozzles in unison relative to a glass sheet supported in fixed position between the moving sets of nozzles or vice versa so that each glass sheet portion was cooled repetitively by one or more discrete blasts in a limited area or a so-called "pass-through" type of tempering apparatus in which hot glass sheets passed between fixed nozzles extending from opposed plenum chambers to direct different blasts of tempering medium, such as air, against each increment of each of the opposite major surfaces of the glass sheets that moved along a path of travel between the opposed sets of nozzles. It is obvious that glass sheets moving through a tempering apparatus along a path of movement provide a different problem from that encountered with a stationary glass sheet supported within an area of limited movement of a pair of opposed sets of tempering nozzles or that met in moving a glass sheet either orbitally or in a reciprocating path within an area defined by fixed nozzles. The solutions involved in providing for the escape of tempering medium after it is applied at cold temperature against the opposite hot glass sheet surfaces or targets that are held in stationary position between nozzles moving relative to the stationary targets or those which occupy repetitive positions relative to the nozzles do not solve the problem of promoting the escape of spent tempering medium applied against moving targets that occupy different positions during the cooling step of a tempering cycle.
It would be beneficial for the glass tempering art to develop a technique whereby tempering medium blasts applied at a cold temperature against the heat-softened surfaces of a continuously moving, hot glass sheet are permitted to flow and escape more readily than permitted in prior art devices in order to facilitate the application of additional cold tempering medium toward the opposite major glass sheet surfaces as the sheet moves through pass-through tempering apparatus to provide an adequate temper in the moving glass sheet.
2. Description of Patents of Interest
U.S. Pat. No. 3,186,815 to Jochim dicloses a glass tempering apparatus designed to temper different portions of the glass to different degrees of temper by providing a separate set of nozzles moveable relative to the remaining tempering nozzles in a direction parallel to the thickness of a stationary glass sheet being tempered. The purpose of this invention is to provide different portions of the tempered glass sheet with different properties that are associated with different degrees of temper.
U.S. Pat. No. 3,294,519 to Fickes discloses apparatus for tempering a stationary glass sheet in which air under pressure is supplied to a pair of opposed plenum chambers and imparted through nozzles having a larger proportion of tempering medium-imparting area per unit cross section area in the central portion compared to that of the portions exterior of the central portion. The purpose of this patent is to increase the flow rate of tempering medium against the central portion of the glass sheet undergoing tempering so as to cause a pressure gradient in the tempering medium parallel to the major surfaces of the glass sheet from the central region to the entire marginal portion of the stationary glass sheet.
U.S. Pat. No. 4,323,385 to Dean W. Gintert and Raymond W. Waksmunski, U.S. Pat. No. 4,314,836 to Samuel L. Seymour, German Pat. No. 1,808,117 to Tilmant and Japanese Patent Specification No. 26320/42 to Amano, all show apparatus for treating stationary glass sheets. The disclosures in the Gintert et al and German documents utilize two concentric areas of nozzles, an inner area of nozzles surrounded by an outer area of nozzles. The nozzles in the inner and outer areas are arranged differently from one another. The Seymour patent provides exhaust areas interposed between nozzles throughout the tempering apparatus to help air blasts to escape in directions parallel to the glass sheet thickness. In the Japanese disclosure, there are three concentric areas of nozzles in which large diameter nozzles are located in the intermediate area, the innermost area is provided with closely spaced small diameter nozzles, and the outermost area is provided with nozzles of intermediate diameter disposed in a different spacing from the nozzles in the other two areas.
U.S. Pat. No. 4,282,026 to McMaster et al discloses tempering apparatus in which a shaped glass sheet is supported on a ring-like mold between upper and lower blastheads that supply cooling air toward the opposite glass sheet surfaces. The carrier for the mold preferably oscillates back and forth between the blastheads to uniformly distribute the impingement of cooling air with the glass. No provision of non-uniform nozzle arrangement is made in this apparatus to facilitate the escape of spent air.
None of these patents disclose any apparatus for tempering glass sheets that move along a continuous path of movement past a succession of nozzles arranged in opposing sets that face the opposite major surfaces of the moving glass sheet that represents a moving target. In the prior art discussed thus far, the glass sheet did not move continuously through a tempering apparatus without stopping. Stopping for the type of tempering provided by the patents mentioned previously reduces the maximum rate of tempered glass sheet production, as it is necessary for consecutive sheets being heated along a conveyor passing through a tunnel-type furnace to wait until a glass sheet is cooled sufficiently at a cooling station before the glass sheet can be released from the cooling station and replaced by a succeeding glass sheet in a series of glass sheets for cooling of the succeeding glass sheet at a rate rapid enough to cause it to develop a temper.
U.S. Pat. No. 4,046,543 to George B. Shields discloses a glass sheet tempering apparatus of the pass-through type comprising slot-type nozzles and means to provide escape paths for spent tempering medium provided in this patent that are different from those provided in the present invention. This patent also provides screens of non-uniform porosity designed to deliver tempering medium at more uniform flow across the width of the path of glass sheet travel.
U.S. Pat. No. 4,119,427 to Revells discloses tempering apparatus of the pass-through type. This apparatus includes cooling means comprising upper and lower blastheads disposed above and below the path of glass sheet travel. Series of tubes arranged in longitudinally spaced sets disposed across the width of the path of glass sheet travel extend from the blastheads to direct opposing streams of cooling fluid toward the opposite surfaces of the moving glass sheets.
The escape paths for spent cooling fluid provided in the Revells patent are similar to those of the Shields patent and comprise open spaces extending transversely across the entire width of the tempering apparatus alternating with sets of nozzles along the length of the path of glass sheet travel. The nozzles are elongated slots in the Shields patented apparatus and of pipe-like construction in the Revells patented apparatus.
To the best of our knowledge, the pass-through type of tempering apparatus of the latter Shields or Revells patents were not constructed to facilitate the escape of spent tempering medium from the center to the sides of the path of glass sheet travel along the major surfaces of the glass sheet. A search of prior art failed to discover pass-through tempering apparatus specially constructed to permit additional cool tempering medium to be applied more readily against the opposite major surfaces of the hot glass sheet, particularly toward its central portion transverse to the path of travel.